Conductors, Insulators and Semiconductors

This lesson is the materials half of the subtest. It connects the atoms lesson in General Science to every component that follows.

The three categories

The category is set by the atomic structure, specifically by how firmly the outer electrons are held. That is why nearly all metals are conductors: metallic bonding leaves outer electrons free to move through the material.

Conductors, in order

Silver, copper, gold, aluminum is the usual ordering by conductivity.

Copper is used for wiring because it is nearly as good as silver and a great deal cheaper. That trade-off is the question, not the ordering: a question asking why copper rather than silver wants cost.

Gold is used for contacts and connectors, not for its conductivity but because it does not corrode. A tarnished contact is a bad contact, and gold does not tarnish.

Aluminum is lighter and cheaper than copper and is used where weight matters, including in overhead transmission lines and in aircraft. It is a poorer conductor per unit of cross-section, so aluminum wiring has to be thicker for the same current.

Insulators

Rubber, plastic, glass, ceramic, dry wood, dry air.

Air is an insulator until it is not. A high enough voltage ionizes it and it conducts, which is what a spark and a lightning strike are.

Water's conductivity comes from dissolved minerals. Pure water is a poor conductor; tap water, sweat and seawater conduct well, which is the practical point.

Dry wood insulates; wet wood does not. The qualifier matters in a question.

Wire gauge

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American Wire Gauge runs backward: the smaller the number, the thicker the wire.

10 AWG is substantially heavier than 18 AWG. That inversion is the most asked fact in this lesson, because every other numbering people meet runs the other way.

Thicker wire means:

• Less resistance, since resistance falls with cross-sectional area.
• More current-carrying capacity, because less resistance means less heating at a given current.

So heavy loads need a low gauge number. A household lighting circuit might use 14 gauge and a large appliance circuit 10 gauge, and putting an appliance on wire that is too thin makes the wire, rather than the appliance, the thing that gets hot.

Too-thin wire is a fire risk, not a performance problem, which is the framing a question usually takes.

Voltage drop along a wire

Every wire has some resistance, so a long run loses voltage along its length. That loss is voltage drop, and it is Ohm's law applied to the wire itself: drop = current x the wire's resistance.

A 100-foot cable with 0.02 ohms per foot carries 15 amperes. What is the voltage drop?

The cable has 100 x 0.02 = 2 ohms, and 15 x 2 = 30 volts lost before the current ever reaches the load.

That is why a long, thin extension cord makes a power tool or motor run poorly: the tool receives less voltage. The fix is a larger wire - a lower gauge number - which has less resistance per foot. A longer run needs a heavier wire for the same load, and aluminum needs a larger size than copper for the same current because its resistivity is higher. Resistivity is a property of the material; making a wire longer changes its resistance, not its resistivity.

Stranded wire, made of many fine strands, is used wherever wire flexes, because it bends repeatedly without breaking; solid wire is stiffer and cheaper. The common insulation on building wire is PVC plastic.

Heat limits how much current a wire may carry (its ampacity). Bundling many current-carrying wires together traps their heat, so each must carry less - which is called derating.

Semiconductors

A semiconductor conducts under some conditions and not others, and that controllability is the entire basis of modern electronics.

Silicon is the important one, with germanium the historical alternative.

Doping is adding a small quantity of another element to change the behavior:

• N-type doping adds material with spare electrons, giving negative charge carriers.
• P-type doping adds material with electron shortages, giving positive carriers, called holes.

N for negative, P for positive - the letters say it, which is a rare kindness in this subject.

A junction between p-type and n-type material is a diode, and the arrangement of three such regions is a transistor. Every device in the diodes lesson is built from this.

Semiconductors conduct better when heated, which is the opposite of a metal. That contrast is asked: a metal's resistance rises with temperature and a semiconductor's falls.

Superconductors

Occasionally named. A superconductor has zero resistance below a critical temperature, which is very low. Worth recognizing the word; nothing more is asked.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Band theory - valence and conduction bands, band gaps - comes up once, as the reason LED colors differ. The loosely-held-electron explanation above is the one asked.
• Superconductor physics beyond the one-line definition is not asked.
• Code tables of ampacity by gauge. No question asks you to look up a number; the questions ask which direction to go.

Where people lose points

Thinking a higher AWG number is a thicker wire.

Saying copper is the best conductor. Silver is; copper is the best value.

Saying gold is used for its conductivity. It is used because it resists corrosion.

Saying pure water conducts well. Dissolved minerals do the conducting.

Saying a semiconductor's resistance rises with temperature. It falls, unlike a metal's.

Fitting a fuse rated for the appliance rather than the wire.

Work one in under a minute

A circuit is upgraded to carry more current. Should the wire gauge number go up or down, and why?

More current needs a thicker wire, so it needs a lower gauge number - 12 rather than 14, for instance.

The reason: a thicker conductor has less resistance, and since heating is current squared times resistance, less resistance means less heat at the higher current.

The gauge number going down while the wire gets bigger is the whole trap, and saying the reason out loud is what keeps it straight.

Where this leads

Doped semiconductors are what diodes and transistors are made of, and wire gauge is what circuit protection has to be matched to.